Production of metal castings

ABSTRACT

A process for the production of a metal casting which has a substantially pit-free surface whereby the metal is cast into a mold such as a block or piece mold which has a layer of particulate active carbon, that is undergoing combustion, covering that portion of such mold which contains or holds said cast metal.

I United States Patent 1111 3,590,903

[72] Inventor Percy Ronald Taylor [56] References Cited g Wales UNITED STATES PATENTS 5. 3,474,851 10/1969 Taylor 164/67 x [22] Filed Mar. 19, 1968 2,893,084 7/1959 E1sermann.... 164/67 [45] Patented July 6, 1971 {73] Assi nee Monsamo chemicals Limited 3,153,826 10/1964 Horton 164/66 X g London England 3,441,078 4/1969 Chandley 164/53 32] Priority Man 3 1967 3,420,291 [/1969 Chandley et a1. 164/66 [33] Great Britain Primary Examiner-J. Spencer Overholser [31 14824/67 Assistant Examiner-John E. Roethel AnomeysRichard W. Sternberg, Roger R' Jones and James J. Mullen [541 PRODUCTION OF METAL CASTINGS No Drawmgs ABSTRACT: A process for the production of a metal casting [S2] U.S. C1 164/53, which has a substantially pit-free surface whereby the metal is 164/67, 164/72, 164/121 cast into a mold such as a block or piece mold which has a [51] lnt.Cl B22d 23/00 layer of particulate active carbon, that is undergoing com- [50] Field of Search 164/72. 53, bustion, covering that portion of such mold which contains or PRODUCTION OF METAL CASTINGS This invention relates to a precess for the production of metal castings, in particular to an improved process for the casting of ferrous metals.

Our British Pat. Specification No. 21 16/66 describes a process for the production ofa metal casting in which molten metal is poured into a ceramic shell mold, and the casting and shell are allowed to cool while surrounded by active carbon in particulate form. By this process, satisfactory castings can be obtained from ferrous metals such as plain carbon steels, low alloy steels and many ferritic and martensitic steels which cannot be cast successfully using ceramic shell molds under ordinary atmospheric conditions, or in certain instances when using other forms of carbonaceous material to surround the shell during casting.

We have now found that the beneficial effects attendant upon the use of activated carbon in the process using ceramic shell molds, for example the avoidance of surface pitting, are also obtained in metal casting processes employing block molds or piece molds. Decarburization of the metal surface is also much reduced or avoided.

The process of the present invention is accordingly one for the production of a metal casting, in which metal is cast into a block or piece mold that contains or is surrounded by particulate active carbon that is undergoing combustion.

In a mold that contains active carbon, the carbon is disposed in one or more cavities within the mold other than the pattern cavity.

The improvements obtained are believed to be attributable to the purging of air from the permeable material of the mold and from the pattern cavity by the gaseous combustion products of the carbon.

In the usual method of operating the process of the inven tion the mold is made and fired in conventional manner (the furnace temperature is usually from 850 to l,l C. but may occasionally be as high as l,200 C.), except that it may be formed with additional cavities for the subsequent accommodation of active carbon.

ln the preferred manner of operating, the active carbon is brought into association with the mold after withdrawal of the latter from the furnace, i.e., the mold is embedded or the cavities filled with active carbon, before the temperature of the mold has dropped below 750 C, or more preferably before the temperature of the mold has dropped below 850 C. Thereafter, at least the minimum time for effective purging should elapse before the metal is poured. Whether or not at the end of this time the metal is poured immediately or later depends largely on what mold temperature is best suited to the particular metal concerned. The mold should not, however, be allowed to cool to a temperature at which effective combustion of the active carbon ceases, the practical minimum temperature in this connection being usually about 500 C.

Since a considerable fall in mold temperature can be tolerated and the typical block or piece mold has a reasonably high heat capacity, the above procedure can usually be operated without recourse to additional heating. lf thin sections are being cast, however, or for any other reason high mold temperatures at casting are required, the mold can be refired for a short period after conjunction with the active carbon.

Optimum operating conditions, including the time required for purging, will, of course, vary according to the size, thickness and geometry of the mold, and to establish such conditions requires a certain amount of experimentation. For the production of castings completely free from surface defects, the combustion of the carbon in situ relative to the mold normally needs to be allowed to proceed for not less than minutes before the mgtal is cast, but as little as l minute may be adequate in certain instances.

Purging can be made more efficient by constructing the assembly so that the preferential path for the discharge of the gaseous combustion products of the carbon lies through the permeable structure of the mold into the pattern cavity. This can be arranged, for example, by sealing the body of the mold (leaving the pouring cup to protrude) into a jacket of vaporimpermeable material, for example mild steel, containing the active carbon, or by capping with impermeable material the carbon-containing cavities of a cavitied mold. Using such an arrangement, it may, under certain circumstances, be possible to allow the mold to cool below 500 C. before casting the metal, since the inert atmosphere generated by the combustion of the carbon at higher temperatures is dissipated only slowly.

Active carbons are carbons characterized by high porosity and correspondingly high surface areas. The production of active carbons usually involves a first stage in which the raw material, for example bone, wood, peat or nut shells, is carbonized, normally by heating in the absence of air. In the second stage the resulting char is subjected to an activation process. Several such processes have been proposed, and one which is used extensively involves controlled oxidation of the carbon with suitable gases. For example steam or carbon dioxide can be used at temperatures of 800900 C., or air can be used at 300600 C. The oxidizing gases remove residual hydrocarbons and other volatile material and cause an erosion of the carbon surface.

Active carbons generally have specific surface areas of at least I00 square meters per gram, and for use in the present invention, active carbons having specific surface areas of at least 500 square meters per gram, and more especially in the range l,0006,000 square meters per gram are preferred. (The surface area is usually determined by gas-absorption techniques based on the procedure of Brunauer, Emmett and Teller). A further preferred feature is that the active carbon should have a residual volatile content not exceeding 5 percent by weight. Active carbons derived from charcoals of vegetable origin have given particularly good results in the present process, especially an active carbon produced by the pyrolysis of coconut shells.

in respect of particle size, we prefer to use somewhat finer material than that indicated as suitable where other forms of carbon have been proposed. Preferably, substantially all the particles should pass through a 16 B.S.S. mesh, and more preferably substantially all should pass through a 30 B.S.S. mesh. At the lower end of the particle size range, material that contains any substantial proportion of particles passing a 200 B.S.S. mesh is rather dusty, and while effective for the process of the invention is inconvenient to handle, and tends to generate dirty working conditions. it is therefore preferred to use carbon substantially all of which is retained by B.S.S. mesh. Commercially, the active carbons are available in various grades corresponding to different ranges of particle size; grades having particles size ranges of -30+80 and 52+120 B.S.S. mesh have been used very successfully. A grade having a particle size range of l6+60 B.S.S. mesh has also been shown to be satisfactory.

While the process of the invention can be used for production of castings of high-chromium steels, its particular advantage lies in the fact that it permits satisfactory casting of plain low-carbon steels, for example BS. 1617A, BS. 3146 CLA. 9; of low-alloy steels, for example those of the Fortiweld" type and type EN36C; and high-carbon, high-alloy, tool steels generally containing around 12 percent CR and other alloys which have poor high-temperature scaling resistance.

A block mold for use in the process of the present invention can be made by a generally conventional technique in the following stages.

A pattern of the article, fabricated in wax or a thermoplastic, is given a primary coating of a slurry containing a finely divided refractory material, such as sillimanite, which is then allowed to dry. The coated pattern is wax-welded to a baseplate of a container, the walls of which are then placed in position round the pattern and sealed to the base either mechanically or by means ofa wax seal.

A slurry made up of a liquid binder, for example a hydrolyzed ethyl silicate or a silica aquasol, and relatively coarse refractory aggregate is poured into the container to submerge the pattern several inches below the original slurry level. A typical grading distribution of the refractory aggregate is 30+80 mesh 65 percent; plus 200 mesh 35 percent.

The slurry is consolidated by vibration and the excess liquid decanted. After drying under ordinary atmospheric conditions for a period which may vary according to the size of the mold and the materials used, from several hours to several days, the mold is heated at a temperature somewhat above the melting point of the pattern material, which when molten is run out leaving the pattern cavity. Finally the mold is fired at a high temperature (typically in the range 950-l,l00 C.) for a period of several hours. The container is usually removed following the heat treatment to dispose of the pattern, but in certain instances its removal after the previous drying stage is possible, while in others it may be left in position until after firmg.

A block mold having cavities adapted to contain active carbon can be made by molding sticks of wax (independent of the primary coated wax assembly) on to the baseplate of the container at appropriate locations. The mold is formed from refractory material in the usual way, and the wax sticks are removed at the same time as the wax pattern.

ln normal practice, metal is cast into the mold immediately or very shortly after the removal of the mold from the firing furnace.

The invention is illustrated by the following Examples.

EXAMPLE! A block mold measuring approximately 9 inches X 8 inches X 6 inches was transferred from a furnace at 1,050 C. into a mild steel box having a base and walls of such dimensions as to leave a gap of inch all round between the sides of the mold and the walls ofthe box. The mold was supported on the edges of metal strips approximately V; inch high welded to the base of the box, and when so supported the walls of the box were approximately A inch above the top ofthe mold.

Active carbon having the following particle size distribution B.S. 481 sieves: Percent 30+40 mesh 43 40l60 mesh a t 45 -60+80 mesh 1O 80-l- 100 mesh 2 was then poured via a funnel to fill the space between the mold and the box and to cover the top of the mold to a depth of about A inch but leaving an asbestos sealing ring round the pouring cup of the mold projecting.

A rectangular cover plate of inch thick Sindanyo having a circular aperture to accommodate the sealing ring of the mold was then placed on top of box within the walls. The small gaps between the walls and the edges of the cover plate and between the aperture in the plate and the sealing ring were sealed with an air-setting refractory cement.

The heat capacity of the mold was sufficient to raise the temperature of the assembly to a value at which partial combustion of the carbon occurred. The temperature of the interior of the mold 20 minutes after its removal from the furnace and after its encasement as described above was in fact 935 C. as measured by a platinum/platinum-rhodium thermocouple. It will be appreciated that under the conditions of encasement described, the gaseous products of combustion of the carbon were forced to percolate into the pattern cavity of the mold thus purging air from the interstices of the mold material and from the pattern cavity itself.

Three minutes after the temperature reading of 935 C. was taken, En4OB steel at a temperature of 1,5 C. was cast into the mold. The castings obtained after cooling and removal of the mold had an excellent surface finish and were free from surface decarburization.

EXAMPLE ll A piece-mold consisting of two parts for the manufacture of an aircraft bracket" casting in a martensitic 13 percent chromium alloy was made in the well-known manner of pouring a slurry ofa refractory in a hydrolyzed ethyl silicate binding agent into a frame surrounding each half of the pattern mounted on a metalplate. The slurry set to a gel state in a few minutes enabling stripping of the green" mold from the pattern to take place, after which each mold-piece was dried and tired prior to assembly.

On one half-pattern-plate, suitably smooth tapered pegs were located so as to mold corresponding blind finger-size cavities in the mold-piece made therefrom,

In the mold-piece made from the other half-pattem-plate, similar diameter holes were drilled in the refractory before firing, (at suitable locations and to appropriate depth from the top surfacefurthest from the pattern).

After firing at l,000 C. the mold-pieces were withdrawn from the furnace; the blind cavities nearly filled with the activated carbon, and the open end of each cavity then sealed with an air-setting refractory cement.

After allowing the assembled mold to cool to a pattern-cavity temperature of about 600 C., weights were added to hold the two halves together and the metal poured in the usual way.

The casting so made had an excellent surface free from ox idation defects and decarburization. Corresponding castings made in identical manner but without using the activated-carbon technique described had the characteristic pitting" defeet commonly found on l3 percent chromium steel east surfaces and a variable incidence of decarburization up to 0.040 inch.

What I claim is:

l. A process for the production of a metal casting which comprises casting the metal into a permeable block or piece mold,

having associated with said mold, other than in the pattern cavity of said mold, particulate active carbon undergoing combustion, and

2. allowing the metal to cool.

2. A process according to claim 1, in which the carbon, in situ relative to the mold, has undergone combustion for a time sufficient to permit purging of air from the permeable material of the mold by the gaseous combustion products of the carbon, before such metal is poured into the mold.

3. A process according to claim 2, in which the metal is a ferrous metal.

4. A process according to claim 3, in which the metal is a plain low-carbon steel, a low-alloy steel, a high-carbon, highalloy tool steel or a ferritic or martensitic stainless steel.

5. A process according to claim 4 wherein the outer surface of said mold is surrounded by particulate active carbon undergoing combustion and characterized in that the body of the mold is sealed into a jacket of vapor-impermeable material containing the active carbon.

6. A process according to claim 4, wherein the active carbon is disposed in one or more sealed cavities within the mold.

7. A process according to claim 4, wherein the particle size of the active carbon is such that substantially all the particles pass through a 30 B.S.S. mesh but substantially all the particles are retained by a 3.55. mesh.

8. A process according to claim 4, wherein the active carbon is one having a specific surface area of at least 500 square meters per gram.

9. A process according to claim 8, in which the active carbon has a specific surface area of from 1,000 to 1,600 square meters per gram.

110. A process according to claim 4, wherein the mold has had the active carbon associated with it at a temperature of not less than 750 C., and the metal is cast after the carbon has undergone combustion for at least I minute but before the temperature of the mold has dropped below 500 C.

11. A process according to claim 10, in which the metal is cast after the carbon has undergone combustion for at least 5 minutes but before the temperature of the mold has dropped below 500 C.

12. A process according to claim 4, wherein the outer surface of said mold is surrounded by particulate active carbon.

13. A process for the production of a metal casting which comprises the steps of l. preparing a block or piece mold which is fired at an elevated temperature 2. surrounding the outer surface of said mold before said mold cools to a temperature of at least 750 C. with particulate active carbon having a specific surface area of about 1,000 to about 1,600 sq. meters per gram, a residual volatile content of not more than 5 percent by weight and a particle size range such that substantially all of the said active carbon passes through a 30 B.S.S. mesh but substantially all is retained on a 150 8.8.5, mesh,

3. pouring into said mold, after said particulate active carbon has undergone combustion for at least 60 seconds, and before said mold has cooled to a temperature below 500 C., a molten metal mixture capable of forming on cooling a metal selected from the group consisting of a plain low-carbon steel, a low-alloy steel, a high-carbon high-alloy tool steel, 21 ferritic stainless steel, and a martensitic stainless steel and 4. cooling the molten metal.

14. A process according to claim 13, wherein said metal is poured after said particulate active carbon has undergone combustion for at least 5 minutes.

15. A process for the production of a metal casting which comprises the steps of l. preparing a block or piece mold which is fired at an elevated temperature,

2. disposing in one or more sealed cavities within the mold other than the pattern cavity particulate active carbon, having a specific surface area of about 1,000 to about 1,600 sq. meters per gram, a residual volatile content of not more than 5 percent by weight and a particle size range such that substantially all of the said active carbon passes through a 30 B.S.S. mesh but substantially all is retained on a B.S.S. mesh pouring into said mold, after said particulate active carbon has undergone combustion for at least 60 seconds and before said mold has cooled to a temperature below 500 C., a molten metal mixture capable of forming on cooling a metal selected from the group consisting of a plain low-carbon steel, a low-alloy steel, a high-carbon, high-alloy tool steel, a ferritic stainless steel, and a martensitic stainless steel, and

4. cooling the molten metal.

16. A process according to claim 12, wherein said metal is poured after said particulate active carbon has undergone combustion for at least 5 minutes.

Patent No. 2,590,005 Dated y 97 Invento Percv Ronald Tavlor It is certified that error appears in the aboveident1fied patent and that said Letters Patent are hereby corrected as shown below:

The portion. of the term of the patent subsequent to October 28, 1986 has been disclaimed.

Signed and sealed this 30th day of November 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents 

1. A process for the production of a metal casting which comprises casting the metal into a permeable block or piece mold, having associated with said mold, other than in the pattern cavity of said mold, particulate active carbon undergoing combustion, and
 2. allowing the metal to cool.
 2. disposing in one or more sealed cavities within the mold other than the pattern cavity particulate active carbon, having a specific surface area of about 1,000 to about 1,600 sq. meters per gram, a residual volatile content of not more than 5 percent by weight and a particle size range such that substantially all of the said active carbon passes through a 30 B.S.S. mesh but substantially all is retained on a 150 B.S.S. mesh
 2. allowing the metal to cool.
 2. A process according to claim 1, in which the carbon, in situ relative to the mold, has undergone combustion for a time sufficient to permit purging of air from the permeable material of the mold by the gaseous combustion products of the carbon, before such metal is poured into the mold.
 2. surrounding the outer surface of said mold before said mold cools to a temperature of at least 750* C. with particulate active carbon having a specific surface area of about 1,000 to about 1,600 sq. meters per gram, a residual volatile content of not more than 5 percent by weight and a particle size range such that substantially all of the said active carbon passes through a 30 B.S.S. mesh but substantially all is retained on a 150 B.S.S. mesh,
 3. pouring into said mold, after said particulate active carbon has undergone combustion for at least 60 seconds, and before said mold has cooled to a temperature below 500* C., a molten metal mixture capable of forming on cooling a metal selected from the group consisting of a plain low-carbon steel, a low-alloy steel, a high-carbon high-alloy tool steel, a ferritic stainless steel, and a martensitic stainless steel and
 3. A process according to claim 2, in which the metal is a ferrous metal.
 3. pouring into said mold, after said particulate active carbon has undergone combustion for at least 60 seconds and before said mold has cooled to a temperature below 500* C., a molten metal mixture capable of forming on cooling a metal selected from the group consisting of a plain low-carbon steel, a low-alloy steel, a high-carbon, high-alloy tool steel, a ferritic stainless steel, and a martensitic stainless steel, and
 4. cooling the molten metal.
 4. cooling the molten metal.
 4. A process according to claim 3, in which the metal is a plain low-carbon steel, a low-alloy steel, a high-carbon, high-alloy tool steel or a ferritic or martensitic stainless steel.
 5. A process according to claim 4 wherein the outer surface of said mOld is surrounded by particulate active carbon undergoing combustion and characterized in that the body of the mold is sealed into a jacket of vapor-impermeable material containing the active carbon.
 6. A process according to claim 4, wherein the active carbon is disposed in one or more sealed cavities within the mold.
 7. A process according to claim 4, wherein the particle size of the active carbon is such that substantially all the particles pass through a 30 B.S.S. mesh but substantially all the particles are retained by a 150 B.S.S. mesh.
 8. A process according to claim 4, wherein the active carbon is one having a specific surface area of at least 500 square meters per gram.
 9. A process according to claim 8, in which the active carbon has a specific surface area of from 1,000 to 1,600 square meters per gram.
 10. A process according to claim 4, wherein the mold has had the active carbon associated with it at a temperature of not less than 750* C., and the metal is cast after the carbon has undergone combustion for at least 1 minute but before the temperature of the mold has dropped below 500* C.
 11. A process according to claim 10, in which the metal is cast after the carbon has undergone combustion for at least 5 minutes but before the temperature of the mold has dropped below 500* C.
 12. A process according to claim 4, wherein the outer surface of said mold is surrounded by particulate active carbon.
 13. A process for the production of a metal casting which comprises the steps of
 14. A process according to claim 13, wherein said metal is poured after said particulate active carbon has undergone combustion for at least 5 minutes.
 15. A process for the production of a metal casting which comprises the steps of
 16. A process according to claiM 12, wherein said metal is poured after said particulate active carbon has undergone combustion for at least 5 minutes. 